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Abstract

Integrated photonic circuits offer the possibility for complex quantum optical experiments in higher-dimensional photonic systems. However, the advantages of integration and scalability can only be fully utilized with the availability of a source for higher-dimensional entangled photons. Here, a novel fiber integrated source for path-entangled photons in the telecom band at 1.55µm using only standard fiber technology is presented. Due to the special design the source shows good scalability towards higher-dimensional entangled photonic states (quNits), while path entanglement offers direct compatibility with on-chip path encoding. We present an experimental realization of a path-entangled two-qubit source. A very high quality of entanglement is verified by various measurements, i.a. a tomographic state reconstruction is performed leading to a background corrected fidelity of (99.45±0.06)%. Moreover, we describe an easy method for extending our source to arbitrarily high dimensions.

Figures (3)

Example of a setup for creating and manipulating path-entangled quNit pairs. a.) N non-linear crystal waveguides (ppLN) are used offering the feature of integration together with a higher down-conversion efficiency compared to bulk crystals [18,32]. All N crystals are coherently pumped by a common pump beam λ split on a NxN beam splitter. With a certain probability a photon pair is created via type-I spontaneous parametric down-conversion (SPDC): λ→λA+λB. Due to the small conversion probability, the possibility of multiple SPDC events occurring in one or more crystals at the same time is negligible. Therefore, coherent pumping of N crystals will result in a superposition of the SPDC event happening in one of the N ppLN crystals. In the following step the SPDC pairs (λA,λB)in each mode (1,2,…,N) are separated by their wavelength using N dense wavelength division multiplexers (DWDM) into the two modes1→(1A,1′B),....,N→(NA,N′B). After regrouping the modes by their wavelength, a path-entangled two quNit state is obtained as given in Eq. (1). b.) Each photon then enters an NxN multiport, realized by a combination of phase shifters and beam splitters. By choosing the appropriate phase (φi) and reflectivity(Ri)settings any arbitrary N-dimensional unitary transformation can be realized [4]. Combined with single photon detection a projective measurement is finally realized (section 2.1).

The experimental setup for creating two path entangled qubits. a.) The pump beam λ is split by a variable beam splitter (BS) into the two modes 1 and 2. The splitting ratio is adjusted by changing the distance between the two fibers using a micrometer screw. Each mode enters a non-linear periodically poled Lithium Niobate waveguide (ppLN) creating photon pairs via spontaneous parametric down-conversion. Cascaded dense wavelength division multiplexers (DWDM) separate and spectrally filter the down-converted photon pairs. Modes 1 and 2 (1’ and 2’) define a path-encoded qubit. This leads to the two qubit path-entangled state. Delay lines (τ) and polarization controller (PC) are used to adjust the arrival time and polarization of each mode. b.) 50/50 Beam splitters (BSA, BSB) and phases (φA,φB). Combined with single photon detection the projective measurement |P(1/2,φ)><P(1/2,φ)| is realized (Eq. (2). Before entering the single photon detectors for coincidence detection (&) the signal and pump beams are separated using a WDM. For further separation an isolator (Iso) is added absorbing 775nm but passing 1550nm. After the WDM the separated pump beam is detected using standard photo diodes (PD). A PID controller uses this signal to stabilize the phase (sec. 3.3).